


Sanster Week 2021

by talkingsoup



Category: Undertale (Video Game)
Genre: Aftercare, Age Difference, Alternate Universe, Biting, Body Dysphoria, Chapter Vary By Rating, Codependency, Crying, Crying During Sex, Cunnilingus, Dark, Dirty Talk, Dom/sub Undertones, Dubious Consent, Ecto-Genitalia (Undertale), Ecto-Penis (Undertale), Explicit Sexual Content, Fluff and Angst, Fluff and Smut, Foreplay, Ghost Sex, Intense, Loss, Loss of Control, M/M, Memory Loss, Neglect, No Aftercare, Other Additional Tags to Be Added, Overstimulation, Past Abuse, Please Check Author's Notes For Chapter Ratings and Warnings, Porn with Feelings, Possessive Behavior, Power Imbalance, Praise Kink, Rough Sex, Sanster, Secret Relationship, Self-Destructive Behavior, Self-Esteem Issues, Shame, Soul Sex, Sub Drop, Unhealthy Relationships, Vulnerability, W. D. Gaster Is Not Related to Papyrus and Sans, Watching Someone Sleep, Workplace Relationship
Language: English
Status: Completed
Published: 2021-02-28
Updated: 2021-03-06
Packaged: 2021-03-12 20:21:53
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 7
Words: 18,048
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29765085
Author URL: https://archiveofourown.org/users/talkingsoup/pseuds/talkingsoup
Summary: 1. Rendezvous/Interesting Results2. Lab/Coffee3. Dark/Sleeping At Work4. Memory/"Hey Look At This"5. Strength/Warm6. Cold/"Oh no, that's not right"7. Surface/AU
Relationships: W. D. Gaster/Sans
Comments: 52
Kudos: 69





	1. Rendezvous (wildly inappropriate)

**Author's Note:**

> Chapters vary in rating from Teen to Explicit. Chapters will be labeled accordingly. Please check the notes for individual chapter tags and warnings.
> 
> 1: Rendezvous  
>  **Rating: Mature  
> **  
>  Warnings: Workplace relationship, power imbalance, age gap

Sans had been telling himself for the past year that it would never work out. It came down to logic. Sans tended to move at a glacial pace when it came to these things, a combination of laziness and a sort of offhand belief that no one would ever really want him anyway--the slow pace would give the other monster plenty of time to realize their mistake and give up. Gaster, on the other hand…well, Sans had no idea about pacing, but Gaster had a sense of propriety the size of the ego he claimed not to have. He would never sleep with a coworker, much less an employee, much less one he was many years older than, much less  _ Sans, _ of all people.

So for awhile, Sans just resigned himself to an unrequited crush. It took him awhile to realize that his lingering stares were being not just noticed, but returned. Then it was lingering touches. Then it was finding excuses to stay late at the lab, or to work more closely with Gaster alone. Then it was Gaster keeping a hand on his shoulder for several whole minutes while Sans read some data off the computer. Then, both of them reeling from some triumphant scientific breakthrough, Gaster had impulsively kissed him.

It had been brief and kind of sloppy, just a quick clack of teeth. What Sans remembered best was the joy on Gaster’s face and in his eyelights, how he’d suddenly looked a thousand years younger and happier than Sans had ever seen him, and how surprised Sans had been that Gaster had any kind of impulsive streak at all.

Gaster had pulled away and apologized, despite the way Sans had held onto the front of his coat. Sans had been sure that would be the end of it, that Gaster would never touch him again afterward. Gaster lasted about two weeks before it began again. Sans was the one to kiss him the next time, more of a test than anything. Gaster hadn’t pulled away.

It was some ridiculous bullshit at this point, Sans thought, straight out of a romcom. Stealing kisses, staying too long in bathrooms and elevators, remaining late after work with the excuse of finishing this or that bit of data or calculation. And it had gotten more intense as well, slowly but surely.

Which was how Sans ended up sprawled across Gaster’s desk, shirt gone, Gaster’s fingers weaving through his ribs. He could feel his soul blazing unseen in his chest, could feel his magic trying to swirl down into his pelvis as Gaster ground against him, kissing him hard. This had to be it. Glacial pace aside, he’d wanted this for months now. From the way he could feel Gaster’s soul humming, Gaster wanted it too.

“Doc,” he whispered when Gaster broke away long enough for Sans to speak. “Come on.”

He leaned up to reach for Gaster’s waist and heard him groan when Sans began to fumble with his belt.

**“Sans…”**

Somehow, Sans managed to get the belt undone. But then Gaster pulled back, grabbing Sans’s wrists and pushing them away.

“Doc?”

**“We can’t,”** Gaster said, breath coming fast. He blinked hard down at Sans, seeming to realize just what position they were both in. He let go of Sans’s wrists and stood back, frowning.  **“I am sorry. I went too far.”**

“Doc, I want it,” Sans said, sitting up as well. He tried to catch Gaster’s eyelights, but Gaster had already turned away, rubbing at his face with both hands.

“I want it,” Sans said again. “You don’t have to stop.”

**“We can’t do this, Sans.”**

“Why not? I mean, yeah, aside from the obvious.” Sans scooted to the edge of the desk, feet dangling well off the floor. “Look, I get it’s kind of against the rules, but--”

**“It is more than just that,”** Gaster said, beginning to pace.  **“I should never have…”**

“Doc, come on.” Sans leaned forward, trying to keep the desperation out of his voice. “If I hadn’t liked it I woulda broke this off months ago. And you know I can be real discreet, so you don’t gotta worry about your reputation.”

**“That--it is complicated, Sans.”**

“More complicated than studying Resets and time travel?”

Gaster made a frustrated sound and turned his back to Sans, massaging his temples. Sans watched him for a moment, his soul cooling and sinking when he realized that Gaster wasn’t going to change his mind. He carefully slid himself off the desk.

“Look, uh.”

Sans rubbed at his bare arm, staring at the floor. He should probably just walk away, just call it quits now. Gaster was right--it was too complicated. Sans was being selfish. No amount of telling Gaster that he  _ wanted _ this had ever worked. And the bottom line was that there was no way that Gaster could want Sans the way that Sans wanted him. He should just walk away now, break it off, and at least leave the Doctor with his dignity. 

Some stubborn voice in the back of his head told him that wouldn’t be good enough.

“Can we at least…talk about this?” he asked softly. “Like adults? Even if it’s just, I dunno. Clearing the air or something. Then if you still wanna stop, we can stop.”

Gaster’s shoulders tensed. He folded his arms, still turned away from Sans. Sans watched him, waiting. After several long moments, the tension seemed to drain out of him. He sighed.

**“I suppose…that is part of the problem,”** Gaster said.  **“That we have never really talked about this.”**

“Yeah. Heh. Not surprising, I guess. We’re both kinda idiots.”

Gaster let out a huff that was almost a laugh.

“Tomorrow’s Saturday,” Sans prompted. “How ‘bout we go out for drinks or something? Maybe that bar in Snowdin we like. Uh, could just--talk this shit out on neutral ground, that way neither of us can pressure the other or something.”

**“You sound almost like you’ve had this planned,”** Gaster said dryly, glancing back at him with a faint smirk.

Sans smiled a little, glad that at least he hadn’t completely ruined everything yet.

“Nah, just good at thinkin’ on my feet.” Sans felt his face go warm as he remembered that he was shirtless and he stooped to pick it up off the floor. “Would that, uh, work for you, though?”

There was a long pause. Sans held his breath while he pulled his shirt back on.

**“Yes. Alright.”** Gaster dropped his arms and finally turned back to him.  **“Seven?”**

“Sounds good.” Sans managed to keep the relief out of his voice. “I’ll see you then.”

  
  
  


Gaster arrived before him, of course. He had secured a corner table at Grillby’s and gave Sans an exasperated look when Sans walked in late, like he had expected as much.

Sans had thought to bring a few three-ring binders so that they could at least ostensibly get some real work done while they were here. That didn’t stop him from giving Gaster a shit-eating grin as he sat down.

“Hullo, Doc,” he said. “Nice spot for a secret date, huh?”

Gaster narrowed his eyesockets, but Sans could see the dusting of pink across his cheekbones.

**“This is not a date,”** Gaster said in a thin voice.  **“This is merely a…”**

He waved his hand vaguely. Sans stacked the binders on the table, flipping the top one open.

“Work meeting?” he offered. “Illicit rendezvous?”

**“Normal rendezvous,”** Gaster said. He made a sound like he was clearing his throat, eyeing the binders with mild approval.  **“Have you eaten?”**

Sans blinked. “Oh. I, uh…literally forgot.”

He was putting on a good show, he thought, but he’d been nervous about this meeting all day. His soul had been buzzing in his chest since last night.

**“Well.”** Gaster sighed and waved down one of Grillby’s waiters. **“At least we had the good sense to meet at a restaurant.”**

They ordered food, and Sans got himself the strongest drink he could think of--for courage, he told himself. They made small talk about work and their most recent findings as they waited for the food to arrive, Sans flipping through the binders and pointing out interesting bits of data.

“Should we, uh…”

**“After we eat.”**

It made sense. Bad idea to talk about heavy stuff while eating. All the same, Sans wasn’t sure he had much of an appetite. The anticipation was murder. Things could either go very well tonight, or very poorly.

Fortunately, the drinks did make things a little easier, and the food was good enough to take his mind off things while they ate. Sans remembered to pace himself after the first drink. He was a lightweight, and getting drunk would probably just make him do something he regretted.

The anxiety hit him again like a truck the moment the waiter had cleared their dishes and taken their after-dinner drink orders.

“So.”

**“So.”**

“I get that you got a reputation to think of, yanno?” Sans made a show of flipping to a different binder. “I get that people would talk if they found out.”

**“It is not just that, Sans,”** Gaster said, folding his hand beneath his chin.  **“I am a great deal older than you.”**

“You’re a great deal older than pretty much everyone,” Sans said, not looking up. “I’m an adult. Have been for awhile. Hope I’ve made it pretty clear that you’ve got my consent.”

Gaster coughed a little at the word  _ consent. _

**“You might not care about it, but I do,”** he said.  **“And with my position as your boss, I cannot help but…feel that I am taking advantage.”**

“I mean, I…I think it’s probably a good thing that you care. One of us should. But we’ve kinda been doing this dance for like…god, a year, I think. So--I know you’re not some creep or cradle-robber. You’re not taking advantage. Hell, I uh--I think it might--be the other way around. Feel like I’ve, uh. Maybe I’ve pushed too hard.”

**“You haven’t,”** Gaster said quickly.

“Okay. Good. You haven’t, either. In case you were worried.”

Gaster didn’t answer, but it was obvious all the same. Sans sipped his drink and loosened his tie.

“So what else?” Sans asked after a long moment. “If it comes down to just--yanno, uh. You not wanting me then, uh. I’d rather you just out and say it rather than leave me hangin’.”

**“That--no. I--erm. That is…not the case, Sans.”**

Sans felt his soul pulse. “That sounded like a double-negative.”

**“Let us just--leave it at that for now.”**

Sans decided not to press. He felt a bead of sweat trickle down the back of his neck.

“It’s not like we can’t keep a secret,” he said. “Been doing alright for the past year.”

**“A few indiscretions at work is one thing,”** Gaster said grimly.  **“Sleeping together is quite another.”**

“Then we don’t do it at work.”

Gaster coughed again and took a very long pull from his drink.

**“And if someone were to see you sneaking to my place?”**

“We frame it as being workaholics,” Sans said, tapping the binders. “People will sure as hell believe that of you, and I can make people believe you dragged me into it. Might even be more believable if we offered to bring some of the other scientists along. I know Al wants to see the inside of your house. And all of that’s  _ if _ someone noticed.”

**“And if you change your mind?”** Gaster’s voice was grave enough that Sans had to look up at him. Gaster leaned forward.  **“If we did this, and then later you changed your mind--what then?”**

Gaster’s eyelights were locked on him, intense and burning. Sans swallowed.

“Then I’d…just break it off,” he said. “We could just stop.”

**“You would be willing to continue working for a monster after he took advantage of you?”**

“Yes,” Sans said, as firmly as he could when Gaster was looking at him like that. “Be awkward, but I’m good at ignoring awkward. ‘Sides, I--I haven’t changed my mind in all this time, so.”

**“Sans, I have been…unkind in the past. Callous, inattentive, demanding. I have hurt people. I do not want to hurt you.”**

“You won’t. And if you did, I’d break it off, like I said.”

Gaster gave him a long, searching look before he finally leaned back, seeming to relax a little.

**“And…your health?”**

Sans couldn’t help bristling. “What about it?”

**“You know I’ve had my concerns.”**

“Jeez, Doc, I’m not made of glass,” Sans said, rolling his eyelights. Almost on a whim, he grinned and gave Gaster a sly look before dropping his gaze back to the binders. “What, you think you could fuck me so hard I’d dust?”

Gaster made a sound like he’d swallowed his nonexistent tongue.

“Man, talk about prideful,” Sans said, grin going wider. “Just cause you got a few hundred feet on me. Still a real  _ tall _ order.”

**“God dammit, Sans.”**

“Be a helluva ride, though, yeah?”

**“Please be serious.”**

“I am,” Sans said, grin fading. “My HP’s got nothin’ to do with this. I’ve told you before, I don’t need you walkin’ on eggshells for me. My magic’s stable now, thanks to you. You don’t hafta worry about hurting me like that. I can take whatever you can dish out.”

Sans didn’t miss the brief, thoughtful look of interest that flashed across Gaster’s face. He felt his soul flip over and hid a blush by taking another drink.

**“You are too reckless for your own good,”** Gaster said, expression neutral again.

“So’re you.”

**“Regardless, at the end of the day, it would be wildly inappropriate,”** Gaster said with an air of finality.  **“As well as breaking several rules of conduct regarding relationships in the workplace.”**

Sans drained his drink and set the empty glass back down. He stared at the surface of the table.

“I’m okay with breakin’ some rules,” he said. “Didn’t get this far by following the rules, after all. I…want this, Gaster. It doesn’t have to be some big thing. Not askin’ for marriage here, heh. Just…some good times now and then.”

**“You deserve better than that, Sans,”** Gaster said, a faint note of desperation in his voice.  **“You deserve someone who can be open with you.”**

Sans doubted that was true, but he let it go. He folded his hands on the table, pretending to be intently studying the assorted graphs in the binder.

“If it’s a no, I understand,” he said softly. “I’m not gonna push. I’ll keep my hands to myself at work. And you don’t have to worry about me snitching or calling foul or anything like that. Or, hell, if you wanna skip the sex thing and just keep it more casual, I’m okay with that too. It’s not a  _ need _ sorta thing for me, just…just a want. So…point is, I don’t wanna pressure you any more than you wanna pressure me.”

And if Gaster didn’t want him, for sex or otherwise, then he wasn’t going to internalize it as Sans just being wholly undesirable. Certainly not. Never.

**“You…** ” Gaster trailed off with a heavy, full-bodied sigh. He let that hang there for awhile, and Sans could feel him staring.

**“You are too good for me.”**

Sans snorted.

**“It is true.”**

“Pfft. Sure, Doc, whatever. Pretty sure that’s my line.”

**“Some ground rules, then.”**

Sans was so startled he jerked his head up, staring at Gaster with wide eyesockets.

**“We need to be more discreet at work,”** Gaster said, his voice as calm and neutral as it had been before.  **“We have both been reckless and unprofessional.”**

“O-Okay…”

**“We tell no one. Including family. In the event that we are discovered, this ends immediately.”**

“Okay…”

**“You have my assurance that in the likely event that this goes poorly, your career will be unaffected,”** Gaster continued.  **“And I need your assurance that if I make you uncomfortable or push too hard, you will tell me so.”**

“Yeah. Yeah, I…” Sans hesitated. He hated making promises, but this was too important. “I will. Promise.”

**“Alright.”** Gaster leaned back with a small sigh, like he was releasing some last bit of tension.  **“Good.”**

“So…it’s a yes?” Sans managed to sound nonchalant instead of desperate. “We’re doing this?”

**“If you are certain you want this, Sans.”**

“I am. I do.” Sans couldn’t help a giddy grin. “I, uh. I’ve liked you for awhile.”

**“Indeed,”** Gaster said, coughing politely and looking away. He tapped a phalange against the table.  **“I feel the same.”**

“Oh.” Sans could feel himself going beet red but he no longer had a drink to hide behind. “Cool.”

Both of them fell silent for long enough that it started to become awkward. Sans kept grinning stupidly at his hands, his mind screaming  _ now what?  _ He couldn’t think. Someone  _ wanted _ him.  _ Gaster _ wanted him. He’d been with several monsters before, sure, but none of them had really…cared. Not like this.

“So. Uh.” Sans lowered his voice, his confidence from before suddenly gone. “I could let my brother know I’ll be back late. Tonight. If you want.”

**“Ah. Tonight. Yes. Yes, I suppose you should.”**

Sans let out a breath he didn’t know he’d been holding. His soul felt full to bursting.

He’d been so certain that this wouldn’t work out.


	2. Coffee (four sugars no cream)

**Summary for the Chapter:**

> Gaster pays attention.

**Notes for the Chapter:**

> Please check notes for each chapter's rating and warnings.
> 
> 2: Coffee  
>  **Rating: T**
> 
> Warnings: Pure fluff, swear words

It started when the coffee machine in the physics department broke. This was the second machine this year. The physicists tended to run their coffee machines straight into the ground, and Sans was possibly the worst offender.

It was nearing the end of the day, and Sans was staring down the barrel of an all-nighter, so his first instinct was to walk over to the engineering department.

“Oh, w-we don’t have one,” Alphys informed him cheerfully. She was already packing up her things to head out for the night.

“What the fuck do you mean you don’t have one.”

“Ours broke four months ago a-and the department head never replaced it,” Alphys said with a shrug. “We h-have an arrangement with the chemistry department.”

“You have an  _ arrangement?” _ Sans demanded. “With those gremlins? What the hell? The last time I tried to sneak their coffee Dr. Drex threatened to dose it with azidoazide azide, which I don’t think is even possible.”

“Oh, he’d f-find a way,” Alphys said, very solemnly. “Anyway, we fix their cell phones at no ch-charge, they give us coffee. Y-You’ll have to come up with something to barter with. You know how chemists are.”

“All physicists ever have to trade are theories,” Sans said, sighing with despair. He wasn’t exactly known for his dignity, but prostrating himself before the chemists seemed low even for him.

“All-nighter?”

“Yeah. Me an’ the Doc.”

“Sucks to be you,” Alphys said, giving him a saccharine smile. “Y-You know, there might be some Sea Tea kicking around--”

“You’re evil. Taking joy in my suffering.”

“J-Just paying you back for when you rearranged my figurines.”

“That wasn’t even a prank, that was just an accident.”

“Still!”

“Alright, alright,” Sans turned away, but not before flipping her the bird. “Have a good night.”

“Y-You too!” she called after him in a singsong. Sans trudged back to the physics department, her evil laughter following him.

This was fine. He’d pulled all-nighters without coffee before. Maybe Gaster would let him squeeze in a nap sometime. And if not, Sans could probably keep himself alert just by staring at Gaster and letting his imagination run. Something about endorphins. 

The rest of the physicists slowly trickled away for the night until Sans and Gaster were left alone. They certainly wouldn’t be the only scientists in the building working overnight, but they would have the basement lab all to themselves while they ran a few stress tests on the machine. Gaster had claimed it was necessary.

**“Alright,”** Gaster said, gathering his things and a huge stack of folders.  **“Thank you again for being willing to stay, Sans. These tests are critical.”**

“It’s no problem,” Sans said with a shrug. “With my luck, though, Paps is gonna burn the apartment down while I’m out, heh.”

**“Hopefully that will not happen. If these tests go well, we may be able to leave around three.”** Gaster peered at Sans as he turned to head toward the secret elevator, giving him a searching look.  **“Are you planning to do this without coffee?”**

“Oh, yeah, meant to tell you sooner. The pot broke. Someone shorted the thing.”

Gaster squinted at him, then looked down at the thermos in his hand, then back up at Sans.

**“That is. Unfortunate. Did you check any other departments?”**

“Engineering don’t have one. Apparently they have a sophisticated barter system with chemistry. I don’t wanna try my luck with ‘em, I think Dr. Drex has it in for me.”

Gaster gave a heavy, exasperated sigh and held the folders out to Sans.

**“Take these and meet me downstairs,”** he said.  **“I will speak with them.”**

“Really? Chemists don’t respect authority, yanno.”

**“I am aware,”** Gaster said and nodded Sans toward the elevator. Sans took the folders and went on ahead.

The basement lab was completely empty. Which was normal, considering that only a handful of scientists even knew it existed. Sans stretched out a kink in his spine and set about turning on all the lights and queuing up the power-on sequence for the machine. The amount of time the damn thing took to power on was part of the problem--just another rough edge they’d eventually have to smooth out if it was ever going to be functional.

Sans plunked down at his workstation and checked the computer for any anomalies or strange readings. Even powered down, the machine occasionally emitted odd little pulses of energy, some of which couldn’t be explained at all. Nothing today, thankfully. It was going to be a normal night. Maybe, by some miracle, Gaster would be able to pry just one cup of that incredible chemistry department coffee from the cold, greedy claws of the chemists.

It was nice of Gaster to even try, he thought. He tried not to let that thought lead anywhere. Gaster had said earlier that they were supposed to be completely professional tonight. No messing around. Sans tended to lose his inhibitions when he was sleep deprived, however. And Gaster never ran out of excuses to touch him. He was a bit handsy with everyone, sure, but Sans knew full well that he got special treatment.

Sans heard the elevator ding and he looked over to see Gaster stepping out of the elevator, two paper cups in his hands and his thermos tucked under one arm. Sans grinned despite himself.

“Holy shit,” he said. “What’d you do, threaten their jobs?”

**“Of course not,”** Gaster said mildly, crossing the room to him.

“Then what?”

**“I told them that I would see to it that all of their centrifuges be upgraded to the most recent model before the year is out.”**

“Daaaamn.”

**“It needed to happen sooner or later.”**

Gaster set both cups on Sans’s desk.

_ “Both?” _

Gaster indicated his thermos.  **“It wasn’t just for you. I had run dry as well. And I need you in top form tonight.”**

“Wow.” Sans beamed and picked up the closest cup. “Thanks, Doc. You’re the best.”

He took a sip. Sans didn’t know what exactly the chemists did to their coffee--they had a whole system set up with beakers and tubes and specialized filters, like a gin still but for coffee. He just knew that they managed to brew a perfect cup every time. The perfect temperature, the perfect strength, the perfect flavor profile.

That wasn’t the only thing that was perfect now, though. The coffee was sweet and dark, exactly the way Sans liked it.

“Hold on.”

Gaster had already started to walk to his own station and he stopped, looking back at Sans with a raised brow ridge. Sans stared at him, both hands wrapped around the cup.

“You--know how I take my coffee?”

**“Four sugars, no cream, yes?”** Gaster said, the slightest note of hesitation in his voice. Like he knew, but wanted to be sure.

“Yeah.” Sans blinked at him. “How’d you know?”

Sans thought he could see a touch of pink high on Gaster’s cheekbones.

**“I simply paid attention.”**

Giddy warmth spread out from Sans’s soul all the way to his fingertips and down to his toes. He couldn’t keep the stupid, indulgent smile off his face.

“I could really kiss you right now,” Sans said, against his better judgment. “Thanks, Doc.”

**“Yes. Indeed.”** Gaster coughed slightly and turned away.  **“Think nothing of it.”**


	3. Dark/Sleeping At Work (to be soft)

**Summary for the Chapter:**

> Gaster fails to deal with his feelings.

**Notes for the Chapter:**

> 3: Dark/Sleeping At Work  
>  **Rating: M**
> 
> **Warnings:** dark thoughts, mentions of sex, watching someone sleep, vulnerability, unhealthy habits

Gaster frowned at the formula he was writing on the white board, fidgeting with the marker’s cap. He shook his head, eyelights flicking between the complex equations. Reinventing the wheel, mathematically speaking, was an almighty pain in the ass--but an unfortunate side-effect of studying time travel.

**“Something’s off,”** he muttered to himself, mind skittering from point to point as he tried to identify the problem. It was getting increasingly harder to think straight these days; increasingly harder to focus.

**“Sans,”** he called over his shoulder,  **“come over here and check my math, would you?”**

When he got no answer, he turned.

**“Sans.”**

Sans was slumped over his desk, head pillowed on his arms. Gaster felt a spark of worry, followed by a bigger spark of irritation. The first time he’d caught Sans sleeping on the job, he’d been justifiably concerned. Another symptom of his low HP, of course, and Gaster couldn’t help but see every moment that Sans was at work as a risk to the smaller monster’s health. Surely HP that low meant that he could just drop dead at any moment.

Sans had  _ politely _ informed him that that wasn’t the case. He just got tired easily, and his stubborn perseverance didn’t help. Gaster was used to it now. Sans didn’t make a habit of it, at least not as much as some of the others seemed to think, but it was still a point of irritation. Gaster needed Sans at his best. And he didn’t like it when people worried him.

Gaster sighed to himself and crossed the room, debating whether to wake Sans up gently or to just kick his chair out from under him. He stood over Sans’s desk with his hands on his hips, peering down at him.

This was the third time this week. Work hadn’t been any more strenuous than usual, so Gaster wasn’t sure what was going on. Perhaps Sans wasn’t getting enough sleep at home? Far be it from Sans to ever have a normal sleep cycle.

Sans’s face was turned to the side. His shoulders rose and fell with his breathing. Gaster could see dark circles under his eyesockets.

**“Sans,”** he said quietly. Well, that wouldn’t do. How was he going to wake anyone up when he was speaking in a near-whisper? Yet he couldn’t bring himself to raise his voice.

It occurred to him that these were the only times that he ever saw Sans asleep. During their occasional--trysts--Gaster never let Sans spend the whole night. He didn’t let Sans fall asleep at all. Better for him to leave when none of the neighbors were likely to see him. Gaster had thought about it--the idea of watching Sans wake up beside him, the odd domesticity of watching someone else go about their morning routine.

It was the kind of softness that Gaster couldn’t allow for himself, and certainly not something he could afford for a--

A…

He was just standing here like some kind of creep, watching Sans’s face as he slept. A normal monster would have woken Sans up by now, or at least would have turned away and gotten back to work. Gaster resisted the urge to reach out and stroke Sans’s face. To see him like this, small and vulnerable, unguarded, completely trusting that nothing could possibly go wrong while he slept…

It wasn’t all tenderness. Gaster wished it was.

He sighed again. Then he reached out and gathered up Sans’s labcoat, draped over the back of his chair. He laid it around Sans’s shoulders, spending a few seconds too long arranging it to cover Sans’s neck. His fingers brushed Sans’s vertebrae and he tried not to think about it.

Then he went back to work.

Sans finally stirred awake an hour later.

“Shit--aw shit.”

Gaster looked over from his station, watching as Sans bolted upright, visibly wincing from keeping his neck in a twisted position for too long. He looked around, eyelights fuzzy.

**“Good morning.”**

“Shit, I’m…” Sans paused, apparently thrown off by the coat draped over him like a blanket. Gaster could see him frowning, no doubt wondering how it had gotten there. “I-I’m sorry, Doc, didn’t mean to conk out like that. How long was I asleep?”

**“Little over an hour.”**

Sans pushed himself upright, stretching and cracking his spine. Gaster could see him blushing from here, halfway across the room.

“Dammit. I’m sorry.”

**“You must have needed it,”** Gaster said, voice neutral.  **“That’s the third time this week.”**

“Yeah, fuck, I know,” Sans said, rubbing the back of his neck. “You can take it out of my pay, or uh, I dunno, I can stay overtime the next few days…”

**“I’ll settle for you telling me what’s going on.”**

Sans blinked at him and finally remembered to shuck off the labcoat, hanging it over the back of his chair again.

“Oh, it’s. Uh, Paps is applying to colleges and I’ve been helping him out. Quizzes and, uh, advice on the essays and such. He keeps saying he’s not even sure he wants to go, that he just wants to join the Guard…” Sans scrubbed at his face with both hands. “Been arguing back and forth. ‘S kept me busy.”

**“I see.”** It made sense. That brother of his was nothing but a nuisance, though Gaster certainly wasn’t going to say that out loud. He finished what he’d been typing on his computer and set the program to auto-run. He got to his feet, heading back over to the whiteboard.

Still, a few college applications and some sibling arguments didn’t seem like enough to cause Sans to lose quite this much sleep. Perhaps the arguments had simply been that intense.

**“That sounds challenging. If you would prefer to take a few days off, I would understand.”**

He heard Sans padding up behind him. Gaster glanced back. Sans looked as tired as he had while asleep.

“Nah, I’m good. I’m good. Won’t let it happen again, yeah? I’ll just drink more coffee.”

Gaster gave a pointed look at the small stack of empty paper cups on Sans’s desk. 

**“I worry you are pushing yourself too hard.”**

Sans grinned. “I never push myself too hard.”

That was a blatant lie. Gaster sighed quietly and turned back to the whiteboard.

**“Go home, Sans. I’d like you to take the rest of the week off. College is important, after all. I will be sure to put it in as sick leave.”**

“Look, I said I was sorry. I swear, I won’t let it happen anymore.”

**“This is not a reprimand, Sans,”** Gaster said, giving him a curious look.  **“You need to take better care of yourself.”**

“I know how important this current round of tests is. We’ve almost pinned down the tachyon cascade frequencies.”

**“We’re not going to solve it in the next two days,”** Gaster said with a small bit of frustration.  **“You’re really turning your nose up at a four day weekend?”**

Sans started to answer, then stopped and looked away.

“I--want to work,” he said, quieter. “You need me. And I like… I mean, work’s pretty much the only time I can be near you.”

Gaster stared down at him. Sans let out an exasperated sigh and rubbed the back of his skull.

“Sorry, that’s cheesy as hell. Alright, I’ll--”

Gaster laid a hand on Sans’s face, silencing him and making him turn to meet Gaster’s eyelights. Sans looked up at him, eyesockets widening in surprise.

Gaster leaned down and kissed him. Sans let him, answering in kind, melting into it like he always did. Gaster took hold of his shoulder and gently pushed him back against the nearby table. Sans made a soft sound into Gaster’s mouth when his spine bumped the edge, and Gaster’s tongue formed against his will so that he could deepen the kiss. Sans’s mouth was warm and tasted like stale coffee. There was a little hum of magic as Sans’s tongue formed. He held onto Gaster’s elbow like he wanted to keep him there.

It was always so easy. Sans never hesitated, never went rigid, never backed off, never told him to stop. He was always soft, always pliant, always willing.

Part of Gaster wished he could be as soft as Sans.

Another part of him wanted to push him down onto this table and take what Sans offered, show him how he really felt when words always failed him. Hear the sounds he made in bed echoing in this empty lab.

And another part wanted to find those boundaries and break them. Crack open this small, weak monster’s soul and see what held it together. See what it took to make Sans tell him no.

He pulled back, breaking the kiss. Sans drew in a slow breath, teeth parted, eyelights fuzzy again. He perched against the table, one hand gripping the edge, looking like he was close to just falling over backward.

“Gaster…”

God, hearing his name in Sans’s mouth…

**“Go home,”** Gaster said again, slowly releasing his hold.  **“If you’re refreshed on Monday…perhaps you can come home with me that night.”**

“Oh.” Sans blushed the way Gaster knew he would, and he tried to cover it with a wry grin. “Bribing me now, huh?”

**“Is it working?”**

“Yeah. Alright, fine. I’ll head home…get some rest, sort out Paps’s stuff. Don’t blow anything up ‘til I get back, yeah?”

**“I make no promises.”**

Sans caught hold of Gaster’s hand and gave it a quick squeeze.

“Don’t forget to get some rest yourself, either. Coffee and Determination can’t keep you going forever.”

**“Of course.”** Gaster knew that full well.  **“Goodnight, Sans. I will see you on Monday.”**

“Yeah. G’night, Doc.”


	4. Memory (sex with a ghost)

**Summary for the Chapter:**

> Far in the future, Sans and Gaster meet in the Void.

**Notes for the Chapter:**

> 4: Memory  
>  **Rating: EXPLICIT  
> **  
>  Warnings: explicit sex scene, ecto-genitalia (including penis), cunnilingus angst, loss/grief, multiple orgasms, memory loss, sex with someone who is invisible (I guess???), age gap
> 
> side note: the formatting change for Gaster's dialogue is just a way I differentiate between pre-Void and post-Void Gaster, as well as the effect the Void has on his voice/font
> 
> additional side note: feel free to read this as some kind of "The Scientist" AU :)

He was in an empty black void, like usual, lying on something soft. It was always like this, the brief time they had before the Reset came, or before Sans woke up. Too brief.

“Doc?”

He felt a hand press gently against his chest, a simple acknowledgement. His grin widened, a little sadly. He never could see Gaster. Only hear him. Feel him. He missed being able to look up at his face.

“There you are.”

Another hand cupped his cheek and Sans leaned into it, covering the hand with his own.

“I AM ALWAYS HERE, SANS.”

“Yeah, I know. Heh. Just worry sometimes.”

“I WILL ALWAYS BE HERE FOR YOU,” Gaster replied from somewhere above him, with the sort of confidence Sans wished he could believe. Another hand rested on his shoulder and Gaster’s thumb worked comforting circles into the clavicle. Sans sighed. He had missed this. He always missed this. The scant moments they had together were getting more and more infrequent.

Sans reached up into the darkness above him until his hand found something soft, almost like fabric. He grabbed on gently, trying to pull Gaster a little closer. Or maybe it was the void itself. Was there a difference anymore?

“Can’t keep meeting like this, Doc,” Sans said with a wry grin. He paused as the hand on his cheek began to caress his jaw. “Heh, we really oughta go on a proper date or somethin’. Dinner and a movie. Or I’ll start thinkin’ you just want me for my body.”

A weight settled above him, familiar and comforting. It felt safe.

“WHAT SORT OF MOVIE. WOULD BE APPROPRIATE FOR A DATE IN THE VOID?”

“Space-themed.” Sans’s breath hitched a little as he felt hands pull his jacket open, fingers grazing the sensitive ribs beneath his shirt. “Stars. Bet you could use some of that in here.”

“I SEE ENOUGH STARS WITH YOU.”

“Pssh. Jeez. That was lame, even for you.”

“HERE I THOUGHT YOU. WANTED ROMANCE.”

Sans stretched out on the softness beneath him, throwing one arm above his head. Like this he could imagine that this was before, that they were simply in Gaster’s bed with the lights out. His other hand pawed at Gaster’s invisible form until one of Gaster’s many hands caught it. Sans laced their fingers together.

“You know me.” Gaster pushed Sans’s jacket away from his shoulder. Sans felt Gaster’s tongue run slowly up the side of his neck. “Mmm. I just…ain’t so good with the mush.”

“I KNOW.” Gaster spoke with ease, despite the tongue against Sans’s neck. “SWEETNESS UNDOES YOU.”

“H-Heh.” One hand pushed up beneath Sans’s shirt; another trailed downward, running along the hem of his shorts, massaging at his hip. “Guess I--I got somethin’ of a sweet tooth, then.”

“SANS.” The hands stilled. “DO YOU WANT THIS?”

He always asked. Sans let out a shaking breath and squeezed the hand still in his own. He sat up, his jacket falling off his shoulders.

“Fuck, I need it, Doc.” He wished he could reach out and embrace him; or just see his face, even once. “I need you. I’ve missed you so much.”

Two hands cradled his face. Sans felt Gaster lean forward, felt the impression of his body against Sans’s. There was the illusion of breath against his mouth, and then Gaster was kissing him. Sans melted into it, conjuring a tongue. Here, they could do anything; even Sans could command his body as he pleased. It was one of the very, very few perks of the Void.

Sans pressed hard against Gaster, trembling as Gaster eased him out of his jacket. Gaster’s tongue swept against Sans’s and he moaned into Gaster’s mouth. God, how long had it been? It felt like years, and yet Gaster was exactly as he remembered. More hands were gripping his hips, encircling his spine, pushing up beneath his shirt to toy with his ribs. Gaster had always been incredible with his hands. Sans gasped as he felt fingers dig into the underside of his sternum, pulling him even closer.

Gaster’s body was only an impression at best. It seemed to melt around Sans, drawing him in.

Gaster drew away from Sans’s mouth so he could start trailing kisses along Sans’s jaw and neck.

“Doc…hhh.”

“HOW DO YOU FEEL?”

“It feels good, Doc.” Sans tipped his head forward, pressing it against whatever part of Gaster he could. “You feel good.”

It almost felt real.

Gaster pushed Sans’s shirt up and shifted his attention to Sans’s ribcage, pressing kisses to his sternum, running his tongue along the delicate ribs. Sans leaned back on one hand and covered his mouth with the other. 

“YOU CAN BE LOUD, SANS.”

“I-- _ mm.  _ I-I always worry I might be making noise in my sleep. Wouldn’t wanna--” Gaster ran his tongue up the length of Sans’s sternum, eliciting a soft yelp from Sans. “W-Wouldn’t wanna wake up Paps.”

“I WANT YOU TO BE LOUD, SANS.”

Gaster’s fingers worked beneath the hem of Sans’s shorts, caressing bone. Sans jerked forward with a warbling sound. A hand slipped down the front of his shorts and Gaster ran a single finger along the pubic arch.

“Y-You’re messing with me, Doc,” Sans said around a gasp.

“THERE IS NO NEED TO RUSH.”

“I know. I know, just…I might wake up or somethin’, and I don’t want to wake up mid-fuck.” Sans sighed. “Like last time.”

Gaster pressed a gentle kiss to his mouth.

“WE HAVE TIME.” When Gaster pulled backward, he slipped two fingers into Sans’s mouth instead, working them around Sans’s conjured tongue. “I LIKE THE SOUNDS YOU MAKE. IT IS SO QUIET HERE.”

Gaster squeezed Sans’s tongue lightly between his fingers. Sans made a lewd sound and closed his mouth, drawing them in deeper, sucking at what had once been bone--now, he wasn’t sure. He bucked against the hand still in his shorts. Gaster was circling the pelvic inlet with his hand, not yet entering. His touches were light, teasing. It was frustrating as hell, but every subsequent touch was more intense than the last. Gaster was building him up, slowly.

He turned his attention back to Sans’s chest, and Sans finally tugged his shirt off, so hastily he almost got stuck. Gaster made a soft sound, almost like laughter. It was as close as he could come these days.

“SO EAGER.”

“Doc, come on.” Sans jerked against Gaster’s hand again, but Gaster still wouldn’t enter. “Y-You’re gettin’ me so worked up. Doc, please.”

Gaster slipped a hand up inside Sans’s ribcage, fingertips dancing over his vertebrae, his ribs, his sternum.

“Ah, Doc,  _ please.” _

“PLEASE?”

“D-Don’t make me beg.”

“PLEASE WHAT, SANS?”

Gaster kissed him again, hard, his fingers pressing harder now against Sans’s pelvis. Another hand had crept around beneath him and was toying with his sacrum, pressing against the holes.

“Mmnn, god, you can b-be a real,  _ hn,  _ bastard sometimes.”

“PLEASE WHAT, SANS?”

Sans reached up and grabbed whatever part of him he could, eyelights bright and desperate as he stared up at nothing.

“Please, Gaster, fuck me. P-Please. I want it. I’m tired of waiting. Please.”

Gaster slid four of his fingers up into Sans’s pelvis. A spasm ran through Sans’s whole body. Desire made his bones hum.

“Ah, god,  _ oh.” _

Gaster’s thumb pressed against the sub-pubic angle, his fingers massaging the inside of Sans’s pelvis.

“HOW DO YOU. WANT IT?”

“Ngh. Ah! I want…hah, I want you to wreck me. Nail me to the floor. I-I want to still feel you when I wake up. I-I don’t know when I’ll--ahh…” Gaster had leaned down to kiss his sternum again, and Sans curled his fingers against him. “I don’t know when I’ll see you again. I want…I want to remember it. Please. Gaster,  _ please.” _

It was more than just physical desire. It had been like this for so  _ long,  _ grabbing whatever few moments they could steal for each other. Sans hadn’t seen his lover’s face in over a decade now. This was all they had.

It had to count. They had to make it count.

Gaster pushed Sans down.

“TECHNICALLY,” he said as he slowly pulled Sans’s shorts down off his legs, “THIS IS NOT A FLOOR.”

Sans gave a helpless chuckle, draping an arm across his forehead.

“R-Really, Doc? Semantics, at a time like this? I’m  _ floored. _ ” He took a few shaky breaths to try and steady himself. His bones were warm down to the marrow already, but he wanted this to last as long as possible. In the real world, he would never have the energy for something this intense. 

He stretched out on the floor, or whatever it was, letting his eyesockets drift closed. He could imagine Gaster’s own eyelights roving over his body, studying his naked bones as if Sans was something truly rare and beautiful, like he had done in the old days. Every time he saw Sans, it was like he was seeing him for the first time. Seeing that look on Gaster’s face had once made Sans embarrassed, because why would anyone ever look at him like that? Now, the memory of it drove him wild.

Gaster’s fingers were still working along his spine, his ribs, and inside his pelvis. There was a soft sound, like shifting fabric, as Gaster moved downward. He peppered kisses down Sans’s spine, over the iliac crest and toward the inlet, moving painfully slowly. Sans grabbed a fistful of void as he realized what Gaster was planning.

“D-Doc, d’you…want me to…?”

Two hands pulled Sans’s legs wider, thumbs rubbing hard against his femurs. Gaster planted a deep kiss right at the sub-pubic angle, sucking on the curve of bone. Sans gave a wordless little mewl. He tried to reach down to lay his hands against Gaster’s head, but there was nothing there. The absence made him want to cry.

A hand took his, and they laced their fingers together again.

“YOU CAN MAKE WHAT YOU WANT, SANS.”

His magic had been roiling in his pelvis, urgent to form. Sans finally let it, sighing with relief as his magic settled into a familiar shape. His pussy was already wet, but Sans didn’t have the time or energy to be embarrassed about it. He’d never been very good at holding back with Gaster.

“I REMEMBER. HOW YOU USED TO TASTE,” Gaster said, almost purring with approval. “I WISH…”

He trailed off. There was no point in saying it, and Gaster had never liked to indulge in sentimentality.

“Please,” Sans whispered.

He felt Gaster’s tongue against him, then his mouth. He arched back with a soft gasp, reaching for Gaster again before he remembered himself. The hand laced in his own tightened its grip, squeezing as Gaster curled his tongue inside. Sans closed his eyesockets and tried to imagine that this was back then, that they really were just in Gaster’s bed, that Gaster was still whole, that Sans still had any faith in things.

“Gaster,” he said breathlessly, fingers clawing at the void beneath him. “Ah,  _ god,  _ I d-don’t know how you can still be so good at that when you don’t--technically have a--”

“SHH.”

Gaster closed his mouth over Sans’s pussy and sucked, drawing a shaky cry from him. The hands on him tightened, holding him in place as Gaster ate him out like it was his job. Sans’s soul pulsed warm in his chest, warmer than it ever felt when he was awake, body jerking and stuttering under Gaster’s ministrations. His plans to try and keep himself quiet had failed. He was gasping and whimpering, just as Gaster had wanted.

He would do just about anything Gaster wanted. That had always been part of the problem. But things were so much different now. Gaster had changed in ways Sans couldn’t even put words to.

“Ah!” Sans cried out, throwing his head back as his magic surged.  _ “Ah, _ Gaster!”

The orgasm rolled through him, and Gaster drew it out with a clever twist of his tongue. He hummed in approval and ran his hands up and down Sans’s femurs, soothing Sans back down. Sans lay in the void, panting and savoring the lingering heat in his marrow.

“ALRIGHT?”

“Yeah,” Sans said, letting his arms fall above his head. “Fuck, you’re still so good at that…”

“YOU GIVE ME PLENTY OF OPPORTUNITY. TO STAY IN PRACTICE.”

“Do I?” Sans asked, draping his wrist over his eyesockets again. He felt Gaster moving over him. “‘S hard to remember sometimes…”

“I KNOW,” Gaster said, and Sans could hear the longing wistfulness in his voice. Sans shuddered out a breath, wanting to assure Gaster that he’d remember this one. But there were no promises here.

“Doc…”

Gaster had repositioned himself, a shapeless, invisible mass covering Sans like a heavy blanket. Sans peeked up into the darkness from beneath his wrist, hoping that he was meeting Gaster’s eyelights…if he even still had them. He bent one knee, spreading himself a little wider.

“Please,” he gasped out as Gaster wrapped around him. “Need you inside me.”

“YOU REALLY ARE AMAZING, SANS.”

Sans felt something like fingers trace the outer rim of his pussy before slipping inward, an easy glide thanks to Sans’s first orgasm. His eyesockets fluttered closed.

“Don’t--say stuff like that…”

“AMAZING,” Gaster said, more firmly, fingers withdrawing. “BEAUTIFUL.”

“Doc--” Sans cut himself off as Gaster entered him in a slow, controlled thrust. He choked out a whine as Gaster hilted himself as deep as he could go, the pressure intense until Sans felt his body open up for his lover.

“Fuck, oh fuck--”

Gaster pulled back, then thrust forward again, hard and deep, just as slow as the first. Sans grabbed desperately at whatever part of him he could, digging his fingers in as his body rocked in time with Gaster’s inexorable pace. The darkness wrapped tighter around him, like Gaster was trying to hold him. Not hard enough to hurt, not loose enough to let go. Gaster wouldn’t hurt him. Never again.

Funny how the Void had changed things between them, how it had taken losing everything to finally figure each other out. They’d been so stupid back then. All their hang-ups, Gaster’s drive and Determination and self-control, Sans’s need to please him, to be useful, to save him. It had all amounted to nothing. Just echoes of pain and resentment. Here, they could be almost real. Almost normal. 

The price hadn’t been worth it. It still wasn’t.

“YOU FEEL SO GOOD, SANS,” Gaster rumbled above him, an earthquake that shook Sans’s soul. “YOU’RE THE ONLY THING I CAN FEEL.”

“Please-- _ Gaster--” _ It was hard to talk, hard to make any coherent sounds, breaths coming out in gasps and cries with every thrust. He clenched around Gaster, wrenching a deep, startled moan from somewhere above him. A dangerous move, almost driving Sans over the edge too soon.

If he was the only thing Gaster could feel, then he wanted to be incredible for him; as incredible as Gaster was being, driving so deep it was like he wanted to reach Sans’s soul.

“I love you,” he choked out, before he could forget again. He felt Gaster jerk inside him, a momentary lapse in control. “Fuck, I love you…”

“SANS…”

More hands, taking hold of his face, angling his head so that Gaster could kiss him. Sans moaned into his mouth and Gaster swallowed the sound.

“I LOVE YOU, SANS…”

Gaster was moving faster now, and Sans could no longer hold back his cries, couldn’t even hate himself for being so loud. His soul burned in his chest, matching the heat from whatever was left of Gaster’s own. Sans arched his back against the floor of the Void, mouth falling open. Gaster lifted Sans’s hips so he could fuck him at a better angle, drawing a broken little mewl from Sans as his cock grazed perfectly against that hidden spot inside him.

“Oh  _ stars, _ Gaster, right  _ there--” _

“SANS.” Gaster said his name like it was the only thing worth saying. “AH, SANS.”

“Please--” Sans could feel heat and pressure building beneath his soul, a star about to go nova. “Oh, please,  _ Gaster,  _ I’m so--”

He couldn’t even get the words out, thoughts scattering beneath the intensity and pleasure.

“COME FOR ME, SANS,” Gaster said through a moan. “LET ME FEEL IT. LET ME HEAR IT.”

The orgasm was slow, a roll of thunder that shook his whole being, and he arched upward to meet it, drawing Gaster in even deeper. His mind went white, his soul sparking like a fuse. He heard himself babbling Gaster’s name.

“YES,” Gaster purred, and the raw satisfaction in that simple word sent Sans into aftershocks. He collapsed beneath Gaster, marrow singing, gasping for breath.

One final thrust and Gaster came with a low, guttural groan. Sans whined as he felt a gush of wet heat deep inside him, there and then gone, as insubstantial as the rest of Gaster. He wrapped his arms around the darkness above and held on. The cold was already settling in. The knowledge that this was about to end.

“Not yet,” he whispered, eyesockets stinging. He buried his face against the nothingness. “Please, not yet. I w-want to stay like this.”

“SANS.”

Gaster held him tight, but not enough.

“DON’T GO.”

Sans let out a sob at the pain in Gaster’s voice.

“DON’T--”

  
  
  


Sans woke up back in Snowdin, mind hazy from sleep and from the Reset. He was still tired--not surprising, since he was always tired. He felt sore, as if he’d been exerting himself. His pelvis was damp.

It must have been a nice dream.


	5. Warmth (love like oceans)

**Summary for the Chapter:**

> Gaster and Sans try something new.

**Notes for the Chapter:**

> 5: Warmth  
>  **Rating: EXPLICIT  
> **  
>  Warnings: explicit sexual content, soul sex, body dysmorphia, self-esteem issues, intense emotions, praise kink, possessiveness, crying during sex, multiple orgasms, sub drop, completely nonexistent aftercare, neglect, abandonment, chapter takes a very abrupt turn near the end, lurid descriptions of emotional intimacy
> 
> This one is intense and a little dark toward the end.

“Your soul’s so much warmer than mine.”

**“Mm?”**

Sans curled languidly against Gaster, nuzzling his face against Gaster’s sternum. The heat against his cheekbone was what he imagined sunlight would feel like. A cheesy thought, but he was too tired to stay sarcastic in the privacy of his own head.

“Your soul,” he murmured again. “‘S so much warmer than mine.”

 **“Ah.”** Gaster shifted his arm a little, the better to hold Sans against him. **“Well, it stands to reason. I have much higher HP than you. And I imagine the Determination affects it as well.”**

Sans thought about untangling his legs from Gaster’s, but that would require moving. He felt like a cat asleep in front of a space heater.

“Makes sense,” he agreed, stifling a yawn. “Can feel it when you come. Like this little…burst of flame or somethin’. Can you feel mine?”

Gaster hesitated before saying, **“No.”**

“Heh. Ain’t there a human saying…holding a candle to the sun or something?”

**“Something like that.”**

“Guess that makes sense too,” Sans said evenly. It was easier to think that it was merely that Gaster’s soul was that much more powerful than his own. Better than the alternative, certainly.

He was quiet for awhile, drifting pleasantly, enjoying the warmth and the way his bones fit so well against Gaster’s. Gaster’s bed was enormous, soft and plush--exactly the sort of bed you’d expect from a monster of his stature. Sans wished he could just fall asleep like this. It would be nice to someday wake up to Gaster next to him. But Gaster usually didn’t even let him stay this long after they were done. He had to savor this.

 **“You know…”** Gaster began, haltingly.

“Yeah.” Sans didn’t let even a trace of wistfulness into his voice. “Guess I should get going. S’getting late.”

**“That’s not what I was going to say.”**

There was an odd note in Gaster’s voice that made Sans crack open his eyesockets and peer up at him. Gaster had a very thoughtful look on his face.

“No?”

**“I was going to ask…but perhaps it is too much.”**

“What is it?” Sans asked, a little thrill running through him at the idea that Gaster might ask him to spend the rest of the night.

 **“Well,”** Gaster said slowly, **“that is, if you wanted to feel my soul more directly…and if you wanted me to feel yours…there are ways of doing that.”**

The shock woke Sans up fully. He pulled back just enough so that he could stare up at Gaster, thunderstruck.

“You…you mean, like…soul sex?”

Gaster met his eyelights. **“Yes.”**

“That’s--isn’t that supposed to be…pretty intense?” Intense, and also extremely intimate. More intimate than Sans had ever thought Gaster could want.

**“Often, yes. You have never done it before?”**

“No,” Sans admitted, unsure whether he should be embarrassed or not. There was a flicker of heat in Gaster’s expression at the admission that Sans pretended not to notice. “I’ve never really…connected with anyone that much. Plus my soul being how it is…”

Sans looked away, suddenly feeling a little overwhelmed and _incredibly_ shy. He knew he had to look ridiculous, probably blushing like a virgin.

“You’d…really want to do that? With _me?”_

Gaster touched his face, gently turning it to look into his eyelights again.

 **“I would,”** he said. **“But only if you are alright with it.”**

“I…I don’t know,” Sans said, hating the hesitation. Soul sex was one of the most intimate things you could do with another monster--the level of trust involved to make contact with something so fragile and powerful, the connection that meant shared thoughts, feelings and sensations. It would mean knowing Gaster on a different level. And it would mean Gaster knowing him.

Sans wasn’t sure if he had really signed up for something that intense with Gaster. Gaster had been keeping him at arm’s length all this time. The fact that Gaster had been the one to offer, that he wanted this with Sans…that _anyone_ could want Sans in such a meaningful way…

It was goddamn intoxicating.

 **“Please don’t feel pressured,”** Gaster said when Sans had been quiet for awhile. **“It--perhaps it is too much to ask. I know that we haven’t really--that is to say, our relationship thus far has been quite casual. We shouldn’t rush these things.”**

“Yeah, I know,” Sans said, letting out a breath. “But I--think I want to try.”

**“Are you certain, Sans? We can wait until you’re ready. I realize the suggestion was very abrupt.”**

_God,_ and that was just as bad--that saying no now didn’t have to be a no forever. That Gaster might be willing to _wait_ for him. No one else that Sans had been with had ever wanted to wait on anything.

“Yeah,” Sans said with a nod. He grinned. “Yeah, I do. And, I mean, if it’s too much, we got our safeword. Right?”

**“Always, Sans, of course.”**

“So--heh, okay. How do we do this?”

**“Here.”**

Gaster sat up further, positioning himself against the headboard. He pulled Sans with him and Sans crawled up onto his lap, facing him. Sans could feel his soul humming with nerves and anticipation. He wasn’t sure how he felt about Gaster being able to see his face while they did this, but it wasn’t like he would’ve been able to hide how he felt anyway. Not with his soul exposed.

It was just hard not to feel a little self-conscious.

 **“It’ll be a little easier like this,”** Gaster explained.

“Okay.”

 **“Then…you simply call out your soul.”** Gaster laid a hand against his own chest, the other catching hold of Sans’s hand. Sans laced their fingers together and Gaster let him.

“Okay…”

**“Don’t try to force it. It is alright to be nervous, but your soul will resist if you are hesitant. Our instinct is to protect the most vulnerable part of ourselves. Merely exposing your soul is an act of trust.”**

“Y-Yeah, I know,” Sans said with a lopsided grin. “I remember sex-ed. Plus I’ve seen plenty of cheesy porn.”

**“Porn is often misleading.”**

“Pfft, yeah. Okay.” Sans took a deep shaky breath, grin fading just a little. Just nerves. He wanted this, had always secretly wanted something like this. Not just the closeness with someone--apparently the orgasms from soul sex were the best in the world. It was just…a lot.

“Give me a second,” Sans said, pressing his free hand against his sternum. “And--don’t laugh when you see it.”

**“What? Why on earth would I laugh?”**

“And don’t pity me, either.”

**“Sans…we really don’t have to do this tonight.”**

“No, I’m good. I want to. Just--heh, haven’t called it out in awhile, either. Haven’t--shown anyone in a long time.”

Gaster squeezed his hand. Sans took another breath and called on his soul. There was the smallest little catch, his soul flinching away from the call just for a moment, before he felt it emerge. A heady feeling came with it, the sense of separation from himself. He drew his hand away from his chest and his soul appeared, hovering above his palm.

The disgust was immediate. He hated looking at it. The soul was too small, too dim, off-white and hazy around the edges. A ragged, pathetic little thing. He watched it pulse and flutter in response to his own self-hatred.

“S-Sorry. I know it’s not much to look at.”

Maybe this had been a bad idea. Gaster was going to freak out at the sight of it, like he always did when he remembered how weak Sans was. He already walked on eggshells for Sans, especially in bed. Sans had told him over and over that Gaster could be a little rougher, but Gaster was too afraid of hurting him.

**“I…”**

Sans stared at Gaster’s chest. He could hear the abrupt hesitation in Gaster’s voice, the surprise. No doubt wondering how Sans was even alive.

 **“I think it’s lovely,”** Gaster finally said.

“Pfft, okay, it’s worse if you lie.”

**“I’m not lying. It shows that you are a survivor. A sign of strength in the face of hardship. I am…as you know, I am not good with words.”**

“It’s okay,” Sans said, smiling again. It was nice of Gaster to try.

**“I am honored to have been allowed to see it.”**

“Okay, okay. Jeez. Laying it on thick, huh.”

**“Hardly.”**

There was a dull thrum of magic, low and vibrating through Sans’s marrow, almost like the pressure in the room had changed. Then sudden warmth as Gaster drew his hand away from his own chest. His soul manifested, the normal shape and size but pulsing with bright energy. Sans stared at it, somewhat awed. He’d seen very few souls in his life, and the sheer power of them was always stunning. He could barely, just barely, make out the faintest outline of red--a reminder of what Gaster had done to himself.

It was beautiful. A sun compared to Sans’s dwarf star.

“Wow…”

With both of their souls exposed, and already so close together, Sans could feel the air around them beginning to hum. His bones tingled. Their magic was beginning to wrap around and cling to them, binding them together. In the bad porn he’d seen, this would be where the lighting changed and some really terrible CGI sparkles would appear around the actors to indicate the increase in magic. But there was nothing visible right now; it was pure sensation, like Sans was being wrapped in a warm blanket.

 _“Wow,”_ he said again, leaning forward a little as the sensations made him feel almost drunk.

 **“You can touch it, if you want,”** Gaster said, voice low and throaty. **“When you are ready.”**

“I’m--not gonna accidentally hurt you like this, am I?”

**“Not without the intent to do so.”**

Sans shifted his hand, hesitating partway there. Gaster trusted him this much--that alone was incredible. He would probably be happy just to stare into Gaster’s soul for a few hours, bask in that warmth and feel this heady magic swirling around them.

 **“Just know,”** Gaster said when Sans’s fingertips were only an inch away. **“The change is very sudden. It may be startling.”**

“Okay,” Sans whispered.

He reached forward and very, very gently, pressed a finger to the surface of Gaster’s soul.

Gaster had been right--the change was very sudden. It was like touching a fork to an electrical outlet, the rush of energy with none of the pain. Magic and emotion crashed against Sans like a wave breaking on the shore, hurried little snippets of thoughts and feelings--desire, anticipation, hesitance. Then an outward flow, the wave returning to the sea and carrying Sans’s own magic and emotions with it, back into Gaster.

Gaster’s body jerked and then went almost slack. His head tipped forward and bonked a little too hard against Sans’s shoulder. A chest-deep groan escaped him.

“Sh-Shit,” Sans gasped. “Is that okay? Should I let go? H-Holy shit, that’s…wow.”

 **“It’s alright,”** Gaster said, voice strained. **“Don’t let go. Your--your emotions are just--more powerful than I expected.”**

“I, fuck, should I be trying to hold back?”

 **“No. It’s alright.”** Gaster gave a shuddery sigh. **“Unchecked emotions can…overwhelm a soul. But as I said, without negative intent…mmf. The worst that would happen is--that I come before I can reciprocate.”**

“O-Oh.”

Sans took a deep breath, trying not to picture what that might look like, trying to rein himself in a little. It really was a lot like an ocean, or how he imagined an ocean to be--the pulse of the waves, Gaster’s soul crashing against him and then drawing back, push and pull. With the next pulse Sans felt the bare edges of something sharp and red, but Gaster was holding it back.

Sans took another deep breath. He had to calm down a little. Gaster hadn’t even touched his soul yet but he was already overwhelmed by contact with such powerful magic and such intensity of emotion.

 **“Mm. You’re getting it,”** Gaster said after another moment, and the approval in his tone had Sans blushing and almost losing his grip. **“Yes…very good, Sans.”**

Sans wheezed through his teeth. He let a second finger press against Gaster’s soul, expecting a second rush of power through him. Instead it was more like the waves coming up around his ankles. He felt pulled in deeper, like maybe he was sinking into the soul itself.

 **“Good, Sans,”** Gaster said again, squeezing Sans’s hand tighter. **“How are--you feeling?”**

“Good,” Sans answered stupidly. He couldn’t think of anything clever to say. “You…you can touch mine now.”

**“Are you sure you’re--ready?”**

“Y-Yeah.” Sans wasn’t entirely sure, but he didn’t want to just sit here edging Gaster to death. He wanted to know what it felt like, Gaster’s clever fingers on his soul, Gaster’s magic surrounding him the way he was surrounding Gaster.

“Please. Just--just be careful with me.”

At this angle, Gaster bent over him, Sans couldn’t see it when Gaster reached out for his soul. But he could feel it, a growing surge of power, like two magnets approaching each other.

Then Gaster touched his soul and the whole world went white.

The first moments were like drowning. The magic pulled him down and under, and when Sans tried to breathe, magic filled his chest instead of air. There was a spark of panic as he lost track of where he was, what was happening. Emotions that didn’t belong to him surged around and into him, too many and too complicated to identify. He tumbled down, deeper and deeper, the pull too strong to even think about resisting.

Then, intent. A warm rush of calm, of reassurance and delighted approval, _you’re alright, I’ve got you._

When Sans came back to himself, only a few seconds had passed. He had slumped against Gaster, gripping his arm like a lifeline, face buried in Gaster’s shoulder. He was shivering, his breathing uneven like he’d just come. He could feel Gaster’s fingers wrapped around his soul, and though Gaster hadn’t moved his hand at all, the pleasure was unbelievable.

“Oh god,” Sans heard himself saying from far away. His voice sounded thin and shaky. “Holy shit…”

Gaster made another of those chest-deep sounds, and there was another flurry of emotion and intent, _you did so good, you’re so good._

**“Good, Sans.”**

“I f-feel it.” Sans let out a sob against his will. The praise was overwhelming, even more than the pleasure.

 **“Are you alright?”**

The words were echoed with more intent, more emotion, inquisitiveness and concern, a desire to protect, to make safe.

“It’s s-so much--Gaster--”

 **“Sans?”** More concern, spilling over into alarm. Sans realized too late that he had started crying, was practically sobbing into Gaster’s shoulder, whole body trembling. Sans tried to speak, to assure Gaster that he was alright, to articulate what was happening to him. He couldn’t, his thoughts too scattered. He closed his hand the rest of the way around Gaster’s soul and tried to push how he felt back into him, hoping he was doing it right. He heard Gaster groan against him, then another inward flow--alarm and worry this time.

**“I’m going to let go--”**

“Don’t,” Sans forced out, almost panicking again. _“Please, don’t.”_

He wasn’t doing it right. He didn’t have his head on straight. He was freaking Gaster out, crying all over him, and Gaster had to think he was hurting him..

“I’m--” Why was it so hard to talk? Even his voice sounded wet. Was he really crying that much?” “I’m--okay. I’m okay. D-Don’t--let go.”

He tried to focus. There was no way he’d be able to say what was happening with words. He had to do it like Gaster had done, with emotion and intent and half-thoughts. He could feel Gaster’s confused worry--Sans had to be overwhelming him as much as Gaster was overwhelming Sans.

He took a breath. As he released it, he thought back to his earlier metaphor, comparing all of this to ocean waves. He tried to let it all just flow outward instead of pushing, instead of trying to narrow his emotions into something simple and easy to explain. This was complicated. It couldn’t be simplified.

_You care. You care about me for real. No one’s ever cared like this. It’s overwhelming. But it’s a good overwhelming. And I’m confused. I don’t know if I deserve this._

**“Oh. Oh, Sans.”**

Understanding chased away the confusion and alarm. Then a withdrawal. Gaster wasn’t letting go, but his emotions and intent began to become muted, softer around the edges. A little easier to deal with. Sans let out a shaky sigh, tears still pouring down his face.

**“I’m sorry. One’s first time can be very overwhelming.”**

“I’m okay.” It was easier to speak now, but Sans’s voice still came out thick and wet. “S’a lot, but I’m okay. S’good. Feel so good.”

**“I want you to feel good.”**

The slow roll of Gaster’s intent made Sans let out a shaky mewl, like Gaster was fucking him slow and sweet. More emotions now, muted but intense--desire, pleasure, fondness, and something…something else that came from deeper, something Gaster was still trying to hold back.

Sans exhaled, his own intent going with it. _Want you to feel good too._

Gaster made a soft, vulnerable little noise that Sans had never heard from him. 

**“Ahh…wonderful, Sans. You’re doing--so well.”**

Gaster’s thumb slid downward along Sans’s soul. _Come for me. For me._

Sans came with a loud cry that he couldn’t even hope to stifle. It lasted longer than Sans thought was even possible, until lights burst in his mind and his knees gave out, making him crumple against Gaster. He felt hands holding him up, keeping him from collapsing all the way. He clung to Gaster, panting through his sobs.

**“You alright, Sans?”**

Sans didn’t think he’d be able to speak the rest of the night. He could barely even breathe. The orgasm hadn’t faded--it had simply dipped into a plateau.

He managed to nod, his grip on Gaster’s arm tightening. Another exhale-- _let me do that for you._

Acknowledgement, deep desire.

Sans slid his fingers along the edges of Gaster’s soul. He felt Gaster’s orgasm before Gaster even made a sound. The soul in his hand pulsed with heat and pleasure, the ecstasy becoming almost a feedback loop that made Sans whimper. Gaster made another sound that Sans had never heard, loud and raw and trembling.

Sans almost lost his grip.

 _I love you._ He felt it pouring out of him, felt Gaster’s body twitch in response. _I love you, I love you, I love you._

Quiet emptiness. Then another surge of emotion, and Sans felt it as Gaster’s control broke. Desire, _want, need._ Approval, satisfaction, pleasure. Possessiveness.

_You’re mine._

Sans’s entire body shuddered. His eyesockets fluttered open, focusing on the red-tinted soul in his hand.

_Mine. Mine._

_Yes. Yes, yes, I’m yours. I love you._

**“Another,”** Gaster ground out. His thumb was moving before Sans could even respond, emotionally or otherwise.

There was nothing in the whole world more perfect than the way Gaster’s thumb pressed into the notch at the bottom of Sans’s soul, like it was supposed to be there.

Sans came apart, his cry almost a scream. His toes curled, his mind went blank. The universe shrank into a point of heat and magic and love. The only people who existed were Sans and Gaster. The only thing that mattered was Gaster’s fingers on him.

He was going to pass out. No, that wouldn’t do. He needed--he needed to--

Gaster’s soul was right in front of his face. He lifted it slightly, the orgasm still wringing him out, making him tremble as he brought the soul to his mouth.

**“S-Sans…”**

_Yours. I love you._

Sans pressed his tongue to Gaster’s soul.

**“Sans!”**

Raw magic spilled into his mouth, heat and ozone. Gaster’s back arched. He pulled Sans to him, crushing their bodies together. For a moment, Sans felt almost like their souls had joined.

The world went white again and this time, Sans stayed there. He felt himself drift, felt the different parts of himself slowly finding their way back together, unhurried. He was warm and exhausted and utterly blissed out.

When he managed to open his eyesockets and have a coherent thought again, he was still lying against Gaster. The connection dimmed and faded, magic and emotion receding into the distance. Sans had a moment to see his own soul again before it disappeared into his ribcage. It seemed a little brighter.

He could feel Gaster breathing. There were a thousand things that Sans wanted to say, but it was still hard to think. He didn’t think he’d ever been this tired in all his life, this satisfied, this…happy.

“Fuck,” he murmured, voice muffled by Gaster’s sternum.

Gaster didn’t answer, but Sans heard his breath hitch.

“That was…so good,” Sans said, feeling slow and stupid. “Didn’ know it could feel like that.”

Gaster let out a soft sigh but said nothing.

Sans shifted his arms enough to let them drape around Gaster, the closest he could get to hugging right now.

“Yer amazin’,” he whispered. “Love you so much.”

**“I know.”**

Sans was too tired to identify the emotion in Gaster’s voice.

**“You love me a great deal.”**

“Yeah.” Sans grinned sleepily.

 **“Sans,”** Gaster said, no trace of emotion in his voice at all now. **“I think we have made a mistake.”**

“Huh?”

Gaster shifted beneath him, starting to extricate himself. Sans tried to hold on, but Gaster easily pushed Sans off, rolling him down onto the bed like Sans weighed nothing. Sans blinked up at him, finally realizing that something was wrong.

 **“I have made a mistake,”** Gaster said, moving toward the edge of the bed. **“We should not have done that.”**

Sans tried to bolt upright, but his arms and legs were too weak.

“Wh-What? Why?” Sans struggled upwards, gritting his teeth until he was at least in a sitting position. “What are you talking about?”

Gaster was getting to his feet.

“Doc, what are you doing?” Cold dread filled Sans’s soul, destroying the last of the warmth. “Did I do something wrong? I--I should have asked before I licked it, I-I didn’t--”

 **“It’s not that,”** Gaster said, standing with his back to Sans.

“Is it because I was crying?” Sans had almost forgotten. He wiped at his eyesockets with both hands, but the tears had long since stopped. “I-I’m sorry, I didn’t mean to, I didn’t think that would happen. You didn’t hurt me or anything, I just got--really overwhelmed.”

 **“No, Sans, it’s not you at all.”** Gaster buried his face in his hands. **“I thought that I could hold myself back. I failed. And I failed to realize just how much you--loved me.”**

“W-What?” Sans started to crawl to the edge of the bed and only succeeded in sinking back into the sheets. He was dizzy with exhaustion and panic. On top of that, he was shivering with cold, and more thirsty than he’d realized.

“But you love me too,” he said, feeling almost delirious. “I felt it.”

**“Not--not in the same way.”**

Sans felt almost like throwing up.

“B-Because of--what I felt from you near the end? Doc, I’m okay with it. I’m okay with it. Look, can…” Sans pressed a hand to the side of his skull, blinking hard to try and stop the room from spinning. “Can we f-freak out about it later? I…I need some water, I’m not… I’m really dizzy…”

**“I’m sorry, Sans. I--should not have pushed you into this.”**

“You d-didn’t, I wanted…” Sans trailed off, vision going unfocused. He sank into the bed again, forgetting why he was struggling so hard to get up. “I wanted it. I liked it. Don’t…don’t do this, okay? I--I need you right now. Don’t leave me like this.”

Gaster was picking up his clothes, moving toward the door.

 **“You may sleep here tonight,”** he said from very far away. **“I will stay in my guest room. When you have your strength back, you should go home.”**

The world was fading. Sans felt tears in his eyesockets again. His soul hurt like it was being squeezed.

“Please…don’t leave me…”

**“I am so sorry, Sans.”**

Sans heard the door close. He sobbed into Gaster’s pillow until he passed out.


	6. Cold (if you're gonna hit me hit me harder)

**Summary for the Chapter:**

> Bad decisions, bad mistakes and a much-needed conversation.

**Notes for the Chapter:**

> 6: Cold  
>  **Rating: EXPLICIT  
>   
>  Warnings: explicit sexual content, ecto-genitalia (including penis), somewhat dubious consent (for both of them), rough sex, extremely unhealthy dom/sub themes, extremely unhealthy ideas about sex, unhealthy relationship all around, major self-esteem issues, bruising, multiple orgasms, pain, dubcon dirty talk, possessiveness, biting, dark shit, overstimulation, loss of control, self-destructive behavior,** aftercare, angst, discussions of unhealthy behavior, discussion of neglect, discussion of abuse, heavily implied past abuse, codependency, shame, serious commitment issues, discussion of power imbalance WRT a workplace relationship, let me know if I should add more
> 
> additional note: the end of this chapter might be more wholesome than the rest of it, but this is still a depiction of an extremely unhealthy relationship, and these two don't really recognize that or attempt to fix it until way after this. these dipshits need therapy

“You need to calm down, Doc.”

**“I am perfectly calm.”**

They were alone in the basement. Sans had urged the others to leave after what had happened. Alphys and Katzen had tried to stay, but he’d managed to talk them out of it. He could handle Gaster on his own.

He kept his hands at his sides, standing very still, careful not to make any sudden movements. Gaster was pacing like a caged animal. The lab was a mess; papers strewn everywhere, broken glass, a single smashed computer. A small dent in the side of the machine from where Gaster had kicked it.

The worst part was that Gaster _sounded_ calm now. He’d stopped yelling. But Sans could see the tension in his shoulders as he paced, the flicker of red in his eyelights. He’d reached the point where the Determination was taking over, leaving him cold, calculating, dangerous.

“You gonna be done soon, at least?” Sans said evenly. “Gonna give me a crick in the neck, watching you pace.”

 **“Then leave,”** Gaster said, voice icy. **“Like the others.”**

“I ain’t goin’ nowhere, Doc.”

Gaster rounded on him, eyesockets wide and devoid of emotion.

**“If you knew what was good for you, you would be gone already.”**

“Pssh. When have I ever known what’s good for me?” Sans let his eyelights wink out and he gave an impassive shrug. “‘Sides, I don’t wanna be the guy who leaves someone when they need me.”

Gaster spun away and kept pacing.

**“We are not talking about that.”**

“No. ‘Course not. Why would we.” Sans’s eyelights reappeared and he grinned. “Point is, you need me right now, so I’m not leaving.”

Gaster pressed both hands to the sides of his head.

**“I don’t need you.”**

“Yeah you do. Who else around here has the balls to tell you that you’re acting like a lunatic? Or to stick around when you’re smashing everything? Or shoving coworkers against walls?”

 **“I--”** Gaster made an impatient noise into his hand. **“I am not crazy.”**

“No,” Sans said, voice gentling. “I know you’re not.”

**“I should not have done that to Dr. Katzen. I let the Determination get the best of me. I am sorry.”**

“You already apologized,” Sans said. “Pretty sure he already accepted, too.”

**“I am calm now.”**

“Sure. Still not going anywhere.” Sans heaved a sigh. “Look, Doc, you’ve been different since--for awhile now. Pent up, like you got all this energy and nowhere to put it. And you’re taking it out on everyone around you. You gotta find a way to blow off some steam. Relax.”

 **“Ah, yes,”** Gaster growled. **“Perhaps I should sit in a garden and drink some tea. Get myself a massage.”**

“Therapy, maybe.”

**“You are being stupid, Sans.”**

“Okay.” Sans looked away, his nerves starting to get the best of him. No, he had to be calm. He had to let this not matter. Gaster had been going downhill for a month now, and this was the only way Sans could think to stop it.

This was the only way that he could still be useful.

“Then if you gotta take it out on someone, take it out on me.”

**“Throw you against a wall? You are certainly tempting me.”**

Sans took a deliberate step closer, forcing himself to look up at Gaster. Fortunately Gaster had stopped pacing again.

“Don’t think that’d be very satisfying. Seeing as I’d be dust after the first hit,” Sans said carefully. “That’s not what I’m offering.”

Gaster stared at him, expression unreadable. Sans took another step toward him.

“We haven’t fucked since--that night. Like you’ve been scared to touch me. Or like you’re ashamed.”

Gaster’s eyesockets widened, but otherwise his expression didn’t change. Sans desperately wished that he knew what Gaster was thinking.

“I get it. It’s fine. We don’t have to talk about it,” Sans said, though it hurt to say. “But you’ve been a mess since then, and… Look, I’m not saying you just need a quickie to relax or something stupid like that, I just mean--I said a long time ago that I’d be there for you, and lately, you haven’t let me. You’ve been pushing me away, and--it doesn’t matter how that makes me feel, what matters is that that’s _dangerous._ Because if I can’t help you stay on the right path then I’m--I’m worried that no one else can.”

Sans took another step. From this close, he could see that Gaster had gone so rigid that he was shaking slightly.

“Just let me help you, okay?”

**“You have no idea what you are asking for.”**

The icy danger in Gaster’s voice made Sans shiver.

“I do, actually,” Sans said, tone dry. “I saw it in your soul. And I’m okay with it. I don’t know how you got the idea that I’ve never met someone with a kink before. I’ve told you a thousand times now, you can--be rougher with me. I’m not as fragile as you think.”

Gaster’s eyelights were burning brighter now. Sans hoped that was a good sign.

“I get that you’re afraid of hurting me,” Sans said, quieter. “And I’m not gonna push you. It doesn’t have to be sex. I just want to help. I want you to let me--”

Gaster reached out and wrapped his hand around Sans’s neck. Not tight, not even firm, just enough for Sans to feel it. Sans went still, fear and arousal flooding his soul. His hands twitched, but he didn’t move to try and break free.

Gaster stared down at him, head slightly tilted with curiosity. Like Sans was nothing more than a lab rat that had exhibited an intriguing behavior.

 **“I could hurt you,”** Gaster said, voice empty. **“Very badly.”**

Sans swallowed, jaw clicking. He hoped he wasn’t making a mistake.

“But you won’t,” he said, meeting Gaster’s eyelights. “Not like that.”

 **“You cannot know that for sure.”** Gaster ran his thumb along the side of Sans’s neck, a soft hiss of bone against bone. **“You might as well be made of glass, Sans. It wouldn’t even take much.”**

 _“You_ don’t know that,” Sans said, narrowing his eyesockets. “Why don’t you let me show you just what I can take?”

Gaster grabbed the front of his jacket with his free hand and shoved him back against the nearest desk. Sans let out a grunt of pain when his spine hit the edge. Auxiliary hands sprang into existence around Gaster and seized hold of him, grabbing wrists and shoulders and hips. A spare hand swept across the desk, scattering papers and pens. Then Gaster pushed Sans down onto his back. Sans didn’t resist, letting it happen, even though he saw stars when his head hit the desk. The hand around his neck tightened.

Gaster loomed over him, infinitely tall.

 **“I could break you,”** Gaster said through his teeth, voice strained again, hand trembling like he was just barely holding himself back.

Sans grinned up at him. “Go ahead and fucking try, Doc.”

 **“What is wrong with you?”** Gaster snarled. **“Why won’t you tell me to** **_stop?”_ **

“‘Cause I don’t _want_ you to stop, Gaster,” Sans spat, idly testing the grip of the hands holding his wrists. They only tightened. “Not the biggest fan of pain, but if you need it, you need it. Stop being so scared to take it.”

Sans saw it in Gaster’s eyelights when he finally let go. He bent over Sans, staring into his eyelights, and Sans felt hands starting to tear at his shorts.

**“I am going to make you regret this.”**

“Like I said,” Sans said, unable to suppress an anxious twitch as the hands dragged his shorts down and tossed them away. “You can fucking try.”

Gaster leaned closer, shifting his grip on Sans’s neck so that he could pull Sans’s collar away, hard enough that Sans heard the fabric tear. Sans groaned as he felt teeth on his clavicle, eyesockets fluttering slightly. He squirmed against Gaster’s hold and Gaster bit harder, until it started to hurt.

Other hands took hold of his femurs and spread his legs wide.

 **“Form it,”** Gaster growled against his neck.

Sans obeyed, breath catching a little as his magic dropped into place. He was sopping wet, his pussy throbbing in time with his soul.

“Yeah,” he breathed as he heard Gaster undo his pants. “Yeah, come on. I want it.”

**“Little fool. You asked for this.”**

“That’s what I keep--” Sans’s chest hitched as he felt Gaster’s cock slide up along his pussy, coating it in his slick. “--telling--”

 **“Stop talking,”** Gaster said, pushing Sans’s head back further. **“Your job now is to scream.”**

Sans gave a sharp bark of laughter, wincing. “Make me, asshole.”

Gaster’s cock breached him in one sudden thrust, hard enough that the desk wobbled. The pain was immediate and exquisite. Sans’s pussy tightened against his will.

“Ghh _hhaa!”_

 **“How obedient,”** Gaster said mockingly. He thrust the rest of the way inward, burying himself in Sans. Sans let out another yelp that was half-sob, squeezing his eyesockets shut to keep from crying. He wouldn’t cry, not from something like this. It hurt, but not nearly as bad as he’d been expecting.

Gaster had overestimated himself, like he always did.

“Fuck,” Sans gasped, voice shaking. The pleasure was right on the heels of the pain, the tight sensation of being filled to the brim. Gaster leaned down the rest of the way, covering Sans with his body, hands still holding him in a vice grip.

Gaster started thrusting, rocking Sans’s body and shaking the desk. The sharp pain of being penetrated so harshly faded into a more manageable burn that had Sans whimpering and clenching around the cock inside him. His body opened easily for Gaster. He’d been a little worried; they hadn’t done this in over a month. It turned out his body remembered.

It felt so _fucking good._ The pain, then the overwhelming pleasure chasing behind, making everything bright and sharp and sweet. He was crying out with every thrust, gasping and mewling. So loud. He didn’t even care enough to be embarrassed by it anymore.

“Fuck, _yes,_ Ga--Gaster--” It was hard to speak through the rough movement. “Just-- _like--that--”_

Gaster bit down on his other shoulder. Sans flinched and hissed in pain, a few tears escaping his eyesockets. The new angle meant that Gaster was pressed against both his clit and his g-spot, and the feeling was _mind-blowing._ Pleasure surged inside him, so abrupt that Sans didn’t even have a chance to hold it back.

The orgasm was intense and brief, making Sans’s legs shake. Gaster kept fucking him through it, driving into him harder and faster. It was almost too much, Gaster stretching him to his very limit.

 **“So tight,”** Gaster said, his face still hidden near Sans’s neck. His voice left Sans breathless. **“You’re mine, Sans.”**

“Yes,” Sans rasped, head lolling against the desk, mouth open. He was pretty sure he was drooling. _“Yes.”_

**“Say it.”**

“I’m yours.” Sans tried to tug his arms free again, wanting to wrap them around Gaster and hold him tight.

**“Louder.”**

Sans felt a hand slip down between them. Two fingers pressed against his clit and Sans jerked like Gaster had kicked him.

“Yours, _I’m yours!”_

The fingers rubbed mercilessly, rocketing Sans back toward his peak. Gaster hadn’t even slowed down.

 **“Look at you. You really do like this, don’t you?”** There was something like awe in Gaster’s voice. **“You like being used.”**

 _“Ah!”_ A second orgasm ripped through him, powerful enough that his whole body spasmed. “Yes, _yes,_ G-Gaster, _please--!”_

Gaster finally released his wrists, bracing his hands on either side of Sans’s head instead. Sans seized the opportunity to reach up and grab Gaster’s shoulders, his fingers scrabbling at bone, leaving tracks. Gaster let out a startled grunt but his pace didn’t slow. The burn was getting worse, starting to overshadow the pleasure. Sans’s magic felt raw and overstimulated.

It was fine. There was the safeword if it got too bad. Gaster seemed too fargone to stop if Sans used it, but that was fine too.

He had other methods.

 **“Filthy little thing,”** Gaster said and Sans shuddered. He had never liked that sort of dirty talk, the kind that made him feel as worthless as he truly was. **“Tell me. Tell me how much you love it.”**

“I love you,” Sans choked out, not hesitating.

Gaster almost stopped short, eyesockets snapping wide. Fury flashed through his eyelights, then conflict, then fear--then nothing, his expression going cold once more.

**“Stupid.”**

_Yeah,_ Sans thought, his body slackening as he started to tire. His arms shook. Gaster’s pace was impossible to keep up with. _Yeah, I know._

“Use me,” he said, breathless, the words punched out of him. “I’m yours. Use me.”

Gaster didn’t want him for him. He didn’t want him for his soul, no matter what he’d said when he’d seen it. This was what Gaster wanted. This was how he could be useful. This was how he could keep Gaster from going over the edge, from killing anyone, from turning into the very evil they were trying to stop.

Gaster obliged, taking everything Sans had. Sans went limp on the desk, arms slipping from around Gaster’s shoulders, his grip failing. Gaster’s thrusts became more erratic, harsher still. Sans felt himself come a third time, gasping out a broken little sound, body twitching.

**“Sans…”**

Sans stared blearily up into Gaster’s face, unable to read the expression there.

Gaster leaned down and Sans tried to brace himself for the pain of another bite. Instead, Gaster kissed the marks he’d left on Sans’s right shoulder, tongue pressing gently to each one. Sans made a soft sound. He was too tired to wonder what this might mean.

**_“Sans--”_ **

Gaster hilted himself, coming hard deep inside. Sans whimpered as liquid heat filled him, one last little burst of pleasure. Gaster stopped at last. Sans loosely took hold of Gaster’s wrists. The auxiliary hands disappeared, one by one.

It felt like hours that they stayed like that, Sans splayed across the desk, Gaster perched above him, both of them breathing hard. Sans stared up at Gaster through dark eyesockets; Gaster wouldn’t look at him.

Sans didn’t try to speak until his breathing had settled.

“Take it out.”

Gaster did so, gently pulling out of Sans. Sans winced at the movement, his magic raw and angry, hissing a little as he felt the fluid inside him shift unpleasantly. Gaster started to stand back, so Sans tightened his grip on Gaster’s wrists.

 _“Don’t,”_ he said, as forcefully as he could when he was this spent. “Don’t leave me this time.”

To his surprise, Gaster stopped. A tension that had been sitting in his soul all this time finally released and Sans sighed in relief.

**“Sans…I’m…”**

There was mounting horror in Gaster’s voice. Sans shook his head, though it was little more than rolling his head back and forth across the desk.

“Don’t apologize, either.” He tried to meet Gaster’s eyelights, but Gaster still wouldn’t look at him. “I told you I could take it.”

There was a long silence between them.

**“What…do you need?”**

“Uh…water,” Sans muttered, remembering to actually take stock of himself. “A snack. Blanket. Needta…needta lie down. Might…sleep for a bit. ‘F I fall asleep, don’t--don’t leave, okay?”

**“…Okay.”**

“If I wake up an’ yer gone, I quit.”

 **“Okay. I won’t leave.”** Gaster paused. **“I should also get you cleaned up.”**

“Sounds good,” Sans said with a faint smile.

**“I’m going to pick you up now.”**

“Mmn.”

Gaster carefully gathered him into his arms and lifted him off the desk. Sans let his head loll against Gaster’s chest, his eyesockets sliding closed.

The next hour was hazy. Gaster brought Sans to his office and lay him down in the cot that he used when he spent nights in the lab. Sans sipped some water, trying to control his shivers while Gaster wiped up the worst of the mess with tissues and a damp cloth. Sans drifted in and out, sleeping for a few minutes at a time. Gaster was attentive in a way Sans hadn’t expected, making sure Sans drank enough water and getting him some chips from the vending machine.

Eventually, Sans could feel his energy returning. He woke up from a short nap to find himself tucked into Gaster’s cot. His first instinct was to look for Gaster; he spotted him only a few feet away, sitting in a chair next to the cot, head in his hands.

“Hey,” Sans said. He was a little hoarse from screaming. His voice really wasn’t meant to go that loud.

Gaster didn’t respond, but Sans saw him draw in a deep breath.

Sans pulled himself upright. He was sore as hell, his spine and pelvis the worst of it. He pulled the sheets down and studied himself. There were bruises up and down his arms and across his pelvis and femurs, as well as bite marks on both shoulders. They were fascinating to look at. No one had ever been brave enough to leave a mark on him before. Not physically, at least. Curious, Sans gingerly touched one of the bruises on his femur. It ached a little, the pain dull and warm. He touched his neck and felt the same. He’d need to check that in a mirror later; maybe wear turtlenecks for a few days.

“Thanks,” he said, pulling the blanket around himself. The lab was so damn cold.

**“What for.”**

“For not leaving. For looking after me.”

Gaster sat up straight, pulling his hands away from his face. He stared at the opposite wall, looking harrowed and utterly exhausted.

“You’re freaking out, huh.”

**“I hurt you.”**

“Not really. Nothing I couldn’t handle.” Sans smoothed a thumb over the ring of bruises around his wrist. “I got 1 HP, Doc. If you’d wanted to hurt me for real, I’d be dead. That’s the whole point--intent. Like you said last time.”

**“I…that doesn’t make it better. The Determination…but I cannot even blame it on that. This ugly part of me has always existed. The Determination has simply…drawn it out. Amplified it.”**

“You’re not crazy and you’re not evil.” Sans let out a faint snort. “I mean, even as kinks go, wanting to hurt someone a little isn’t even that far out there. And at the end of the day, I offered. I wanted it. I had a good fucking time, Doc. Wore me out…don’t think I have another of those in me anytime soon, but…man. No one’s ever fucked me like that. It was wild.”

 **“It is not just that.”** Gaster shook his head and folded himself in the chair, draping his arms between his knees. **“It is not just that I hurt you. It is that you felt obliged to do that for me. That you felt obligated to…submit yourself to whatever I might have wanted to do. You promised me, Sans, back when we started this, that you would tell me if you felt uncomfortable or that I had pushed too hard.”**

“Exactly. And I haven’t felt either of those yet. Funny how that works.” Sans glared at Gaster, wishing the older monster would just _look_ at him. “I could have used the safeword and I didn’t. And if you still didn’t stop, I could’ve _made_ you stop.”

 **“You--you thought there was a chance that I wouldn’t respect our safeword?”** Gaster asked incredulously.

“Wouldn’t be the first time someone didn’t.”

**“And yet you still…?”**

“How the fuck do you think I’ve lived this long, Doc?” Sans snapped, patience fraying. “You said I was a survivor. You don’t survive unless you keep an ace in your back pocket. And I’ve got ‘em _in spades,_ heh.”

 **“My magic is much more powerful than yours.”** Gaster still wasn’t looking at Sans, but he was frowning now. **“If I had wanted to--”**

“But you _didn’t_ want to, that’s the whole point. You’ve got it in your head that you’re this awful person cause you’ve got a bit of a mean streak, but you _didn’t_ want to hurt me bad, and I felt how you got more careful when you saw how tired I was getting. And--it’s not about magic, or about power. It’s about knowing how to dodge. It’s about always having a way out. A shortcut.”

**“That does not make any sense.”**

“Yeah. I know. It’s hard to explain. Just--I mean it when I say I could’ve gotten outta there if I thought I was in real danger. I know you think I’m just this weak little thing, that I’m just made of glass, that you could break me if you breathed on me too hard. But I got news for you, Doc, I was broken long, _long_ before I ever met you. And you know what? Glass is more dangerous when it’s broken.”

Gaster directed an exasperated sigh toward the floor.

**“How poetic.”**

“That one’s a freebie,” Sans said, shrugging. “Didn’t even practice.”

**“You are not broken, Sans. Not as much as you seem to think.”**

“Whatever. Agree to disagree.”

Gaster finally looked over at him, but he didn’t look Sans in the face. Instead, Sans saw his eyelights trailing over every bruise and mark he’d left on Sans’s body.

 **“I still hurt you,”** he said, sounding miserable. **“I used you.”**

“I asked you to.”

**“Why?”**

“Because that’s what you want.” Sans grinned bitterly. “You don’t want _me._ You freaked out when you saw how much I loved you. Just like you freaked out over the idea of hurting me. You don’t want someone who loves you. You want something useful. And I wanted to be useful to you.”

**“I--Sans, no, that isn’t--you think I think that little of you? I’m not…I do love you, Sans.”**

“Then why did you leave me like that?”

Gaster started to look away, ashamed. Sans slammed his fist against the wall.

“Look at me, you coward!”

Gaster’s head snapped up and his eyelights finally met Sans’s.

“You just--” Sans’s voice came out choked, like he was going to start crying. No, absolutely not. He crushed it down and tried again. “You just l-left me there. You used me up and _left me._ D-Do you have any idea what that felt like? I was so shaky, so tired, I felt _sick._ I felt like _fucking garbage._ You saw the real me and it scared you so much that you just--you _threw me out,_ like I was a fucking used condom. You think--you think some rough sex and some bruises is _anything_ compared to that? I’ve…I-I don’t have the best history, Doc, people have messed me up before, but no one’s _ever_ hurt me like that. I can _take_ someone being rough with me, I can’t…I _can’t_ take _that,_ Gaster. You don’t have to love me, but you h-have to…you can’t _do that_ to someone. You can’t--take someone’s soul and d-decide a minute later that you don’t want it anymore.”

Sans ground the heels of his hands against his eyesockets to keep the tears from falling. God, he was so pathetic, so disgusting. No wonder Gaster didn’t want him.

Gaster hadn’t looked away. He was staring, openly horrified, openly shocked.

**“Sans…god, I--I didn’t--I thought I was doing the right thing, I didn’t realize--I thought I would make it worse if I stayed, I thought it might break you, I didn’t know you were--I didn’t see it, I refused to see it, I was too focused on--I was too ashamed. It’s no excuse. It’s inexcusable. I’m sorry, Sans, I am…I am so sorry.”**

Sans sucked in a shaky breath, then another, trying desperately to steady himself. He hadn’t expected Gaster to apologize. Not that it really mattered. This time Sans was the one to look away. He stared emptily at the floor, eyelights dim. He shivered a little and pulled the blanket tighter.

“Whatever. It’s fine. Not like I didn’t already know what I’m worth. Not like I’m ever gonna let you see my soul again. You can do whatever you want--fuck me however you want, break up with me, fire me, I don’t care. Just…don’t do something like that to me ever again.”

 **“I won’t, Sans. I promise I won’t.”** Gaster let out a shaky breath and rubbed at his face with both hands. **“I’m not going to fire you, either, of course not. And I don’t--want to break up. I--it is terrible to say it, but I love you. I do love you.”**

“I know you do,” Sans said softly. “That’s what hurt the most.”

 **“It scares me, Sans.”** Gaster whispered it like he was confessing to some terrible crime. **“That I love you. That you love me as much as you do. It scares me.”**

“Why? Why does it scare you so much?”

**“Because I don’t deserve it. Because I may hurt you more. Because I need to focus on the work. Because--every day, I feel another piece of me dissolve. I do not know how much time I have left, Sans. The Determination…our work, the machine, the Resets…there is a precipice ahead, and I do not know the shape of it, and I do not know how to avoid it. I only know that I am going to fall. I do not want to drag you down with me.”**

Sans gripped the edge of the bed and leaned forward, as close to Gaster as he could without tumbling to the floor.

“I’ll stop it,” he said. “I’ll save you.”

**“You can’t, Sans. All that love in your soul… It won’t save either of us. And I am so afraid of what that will do to you.”**

Sans pushed himself to his feet, slowly, wincing the whole way. He wobbled as he stood, bracing a hand on the wall for support, pulling the blanket with him. Gaster watched with sorrowful eyelights as Sans crossed the few shaky steps it took to reach him. Sans laid his hand on the side of Gaster’s face and saw tears rise to Gaster’s eyesockets.

“We’ll figure it out,” he said. “Whatever happens, wherever you end up. I’ll bring you back. I won’t let go.”

Gaster reached up, laying his hand over Sans’s.

**“It will hurt you.”**

“I can take it, Gaster.” Sans turned his hand over so that he could lace his fingers through Gaster’s, holding on tight. “I won’t let go if you don’t.”

Gaster made a broken sound and squeezed Sans’s hand. He smiled suddenly, like it was the only thing he had left.

**“I don’t deserve you.”**

“Yeah, well. Same.” Sans smiled despite himself. “I think we’re both kinda fucked up.”

Gaster drew Sans’s hand forward, pressing his forehead to Sans’s knuckles. Sans felt him shaking.

**“May…may I hug you?”**

“Yeah.”

Gaster pulled him into his lap, holding Sans tight to him. Sans held on, wrapping the blanket around both of them, tucking his legs up and cuddling close. All at once he felt warm again.

They held onto each other as tightly as they could, breathing together.


	7. Surface (beautiful)

**Summary for the Chapter:**

> They've made it.

**Notes for the Chapter:**

> 7: Surface  
>  **Rating: G  
>  Warnings:** mild body horror, unrepentant fluff

“Here we go,” Sans said, grinning as he spotted their destination. “It’s just up this hill. You doin’ okay, G?”

**_Yes,_ ** Gaster answered, his hand movements slow and ponderous.  **_Think I am alright._ **

“You want me to help you up there?”

He saw the unsteady lights in Gaster’s sockets shift to the top of the hill, then back down to Sans’s face.

**_Yes._ **

Sans smiled and took hold of both of Gaster’s hands, turning his back toward the hill and starting to very slowly walk upwards. Gaster’s hands melded into his hands but Sans had gotten used to the putty-like sensation. It had been a little off-putting at first, but the only thing that mattered was that they were able to touch again at all.

Gaster slid his way upwards, following Sans’s lead. He was getting better every day. He was still getting used to physical movement, the same way he was still getting used to linear time. Inclines were particularly difficult. Sans would have picked an easier spot, but Gaster had said that he wanted the practice.

“You’re doing great,” Sans said when they were about halfway up. “See, we’re almost there. It evens out here a bit, should be a little easier.”

It was evening, the air warm, the crickets singing, fireflies starting to rise into the air. Sans watched Gaster’s eyelights tracking the movement of insects and birds in the sky, like he was trying to memorize everything around him. His amorphous lower half spread across the ground, little tendrils spreading out to weave along blades of grass and white blooms of clover. Sans loved the Surface, noticed new things about it every single day, but for Gaster it had to be some kind of miracle. Weight and mass, movement, color, the shape of things. There had been days early on when Gaster had needed to stay in a small dark room, shivering as he held onto Sans, too overwhelmed by it all.

“Here we are,” Sans said, beaming as they finally reached the top of the hill. He stopped for a moment to let Gaster rest. Nearby there was a blanket laid out in the grass, complete with a small basket. Sans’s telescope was already set up.

“Still doing okay?”

Gaster’s perpetual, lopsided smile twitched upward slightly.

**_Yes,_ ** he signed, his hands remembering how to speak even if his voice was still gone.  **_I’m alright. Excited._ **

Sans gave a bright laugh, pleased. “Me too. I’ve wanted to show you this for so long.”

Sans led him over to the blanket, Gaster sighing with relief as he could finally settle into an almost seated position, his lower half going still. Sans sat down beside him, dragging the basket closer. Papyrus had insisted on preparing a few snacks for them when Sans had told him his plan.

“Hungry?”

**_Not yet._ **

Gaster shifted closer to him, and Sans took the invitation to press into Gaster’s warmth. He felt a little like a beanbag chair, impossibly comfortable. He wasn’t going to tell Gaster so, however. Gaster was still a little ashamed of how his body had changed.

They enjoyed the silence for awhile, listening to the buzz of insects and the call of birds. A perfect summer night.

“Meteor shower won’t start for about an hour,” Sans murmured, tipping his head against Gaster’s. “But it’s already so beautiful, isn’t it? Look, see that one?”

He pointed upward.

“That’s Jupiter. We’ll be able to see the cloud stripes later, once it gets a bit darker. A few of the moons, too. And that red one, that’s Mars. Oh, and look, you can just see the Milky Way starting to come out.”

Gaster followed his gaze, sockets wide, eyelights sparkling.

**_Beautiful,_ ** he signed after a few minutes of taking it all in.  **_All so beautiful._ **

“Yeah,” Sans said, snuggling against him. “It is.”

**_Love you, Sans._ **

“I love you too, Gaster. I’m so glad you’re here with me. I’m so glad I got you back.”


End file.
